This invention relates generally to electric immersion heaters for fluid reservoirs, and more particularly to an immersion heater with carbon-based, semi-conductive heating element encapsulated in a thermoplastic sheath and diesel exhaust fluid storage systems incorporating the immersion heater.
Selective Catalytic Reduction (“SCR”) is increasingly used in diesel driven vehicles to meet emissions standards, such as the Euro V or Euro VI standards for road-driven vehicles in Europe, or the EPA Tier IV standards for off-road diesel engines of all sizes used in a wide range of construction, agricultural and industrial equipment in the US. Typically, the SCR vehicle has a urea tank, separate from the fuel tank, which is used to carry an operating fluid such as an automotive urea solution (“AUS”) or diesel exhaust fluid (“DEF”), (hereinafter both or either will be referred to as “DEF”). DEF is a 32.5% solution of high purity urea in de-mineralized water. DEF is typically stored in a urea tank of an SCR vehicle and is sprayed into the exhaust gases of the vehicle in order to convert oxides of nitrogen into elementary nitrogen and water.
Problematically, DEF freezes at a temperature of approximately −11° C. (11° F.). In order to ensure this method of reducing emissions in an SCR vehicle remains effective, the DEF needs to be maintained in a liquid state to allow injection.
Thus, in SCR systems, fluid transfer lines and tanks containing DEF must be heated to allow the SCR system to function at temperatures below approximately −11° C. (11° F.). Thawing occurs at a much slower rate in a tank than in a transfer line, because of the greater fluid volume, leading to slow activation of the system. Current SCR systems use engine coolant circulated through a copper coil in the DEF tank to thaw the frozen DEF. The time required to thaw the DEF is determined in part by the time for the engine coolant to warm up after the engine is started, and that is typically the rate-limiting step delaying the startup up of the emissions control system. In addition to lack of temperature control of the coolant, the cost and complexity of the system is increased by the need for coolant lines running to and from the DEF tank.